1. Field of the Invention
The invention relates to a wavelength measuring system for measuring the wavelength of a light wave oscillated, for example, in a single mode.
2. Description of the Related Art
In the case of a DFB--LD (Distributed Feedback--Laser Diode) light source and a DBR--LD (Distributed Bragg Reflector--Laser Diode) light source, for oscillating a light wave in a single mode, drift in the wavelength of the light wave occurring over the long term has recently become a matter for concern. Accordingly, with a high density WDM (wavelength division multiplex) system, it has become necessary to make measurement of the wavelength of a light wave oscillated by a light source from time to time as necessary for control of the wavelength.
Meanwhile, an external optical cavity type wavelength tunable light source with the use of a diffraction grating has recently been put to practical use, and is now in widespread use for measuring the wavelength characteristics of optical components, and so forth. The external optical cavity type wavelength tunable light source has an advantage in that a wavelength can be set optionally in the wavelength band not shorter than 100 nm, however, on the other hand, it has a problem of susceptivity to external influence, in particular, the laser wavelength of a light wave emitted therefrom undergoing changes due to variation in temperature. In addition, following demands for higher precision with the high density WDM system, the tunable light source is required to have higher precision in the laser wavelength thereof.
Accordingly, there has arisen need of a wavelength measuring system for measuring the wavelength of a light wave oscillated, for example, in a single mode to determine a laser wavelength at a light source. Among conventional instruments for determining a laser wavelength at a light source, there are a spectrometer using a diffraction grating, a light wavelength measuring system utilizing the Michelson interferometer, and so forth.
FIG. 10 is a block diagram showing a rotational type spectrometer, typical of conventional and common spectrometers, using a diffraction grating.
The rotational type spectrometer comprises an input fiber P1 as a light input part, concave mirrors P2, P4, a diffraction grating P3, a slit P5, an optical receiver P6, a signal processor P7, a rotational mechanism P8, a driving circuit for rotation P9, and so forth. A light beam for measurement, radiated from a light source (not shown), is allowed to fall on the rotational type spectrometer via the input fiber P1, and the intensity of the light beam passing through the slit P5 is detected while signals received indicating changes in the intensity of the light beam, caused by rotating the diffraction grating P3, are analyzed, thereby determining the wavelength of the light beam for measurement.
More specifically, the light beam for measurement, radiated from the light source (not shown), falls on the rotational spectrometer through the input fiber P1, and the concave mirror P2 turns the light beam outgoing from the input fiber P1 to parallel light rays and reflects same toward the diffraction grating P3.
The diffraction grating P3 is fixedly attached to the rotational mechanism P8, rotatable by signals outputted from the driving circuit for rotation P9, such that an inclination thereof can be adjusted.
The diffraction grating P3 is an optical element whereby a light ray at a wavelength selected by the angle of incidence q of an incoming light beam is reflected at the angle of reflection b according to the following equation (1); EQU .lambda.=d/m.(sin .THETA.+sin .beta.) (1)
where l is the wavelength of the incoming light beam, d the distance between grating lines of the diffraction grating P3, m the order of diffraction at the diffraction grating P3, q the angle of incidence of the incoming light beam falling on the diffraction grating P3, and b the angle of reflection of the light ray reflected from the diffraction grating P3.
Accordingly, the parallel light rays incident on the diffraction grating P3 are caused to reflect a component thereof, at a wavelength determined by the angle of incidence q and the angle of reflection b as represented by the equation (1) above toward the concave mirror P4. The concave mirror P4 causes light rays after spectral diffraction by the diffraction grating P3 to be condensed at the optical receiver P6, and the optical receiver P6, in turn, outputs an electric signal corresponding to the intensity of condensed light rays at the wavelength, received thereby, to the signal processor P7.
The signal processor P7 finds the wavelength of the light beam for measurement on the basis of an optical signal from the optical receiver P6, at the wavelength selected by the diffraction grating P3, and a wavelength signal from the driving circuit for rotation P9.
With the wavelength measuring system utilizing the Michelson interferometer, another of the conventional wavelength measuring systems, the wavelength of the light beam for measurement is found through Fourier transform of an interference waveform obtained by the Michelson interferometer.
Another spectrometer using a diffraction grating and an optical receiver array (PD: photodiode array or CCD: charge couple device) has recently been developed.
FIG. 11 is a block diagram showing a spectrometer using a diffraction grating p12 and a PD array P13.
The spectrometer comprises an input fiber P11 as a light input part, a concave diffraction grating P12, a PD array P13, a signal processor P14, and so forth. A light beam for measurement, radiated from a light source (not shown), is allowed to fall on the concave diffraction grating P12 via the input fiber P11, and after spectral diffraction, received by the PD array P13, thereby determining the wavelength of the light beam for measurement.
More specifically, the lightbeam for measurement, radiated from the light source (not shown), falls on the spectrometer through the input fiber P11, and the light beam outgoing from the input fiber P11 falls on the concave diffraction grating P12. The concave diffraction grating P12 is fixedly attached to an optical bench (not shown) so as to form the angle of incidence q against the optical axis of incoming light rays, and not rotatable.
The incoming light rays incident on the concave diffraction grating P12 are condensed at the PD array P13, but the position of light condensation varies depending on the wavelength of the respective incoming light rays. Condensed light rays at various wavelengths, received by respective optical receiver elements in the PD array P13, are converted into electric signals corresponding to the intensity of the respective condensed light rays by the respective optical receiver elements, and the electric signals are outputted to the signal processor P14. The wavelength of the light beam for measurement is found on the basis of optical signals at respective wavelengths from the PD array P13 by operation of the signal processor P14.
On one hand, the conventional spectrometer using a diffraction grating, and the wavelength measuring system utilizing the Michelson interferometer have an advantage in that wavelengths in a wide wavelength range can be measured. On the other hand, however, these have a problem of lower reliability over the long term because they comprise movable mechanical components. Further, with these instruments, an optical system needs to be enlarged to enhance a wavelength resolving power, and consequently, it is difficult to reduce a size of an instrument in whole.
Meanwhile, the conventional spectrometer using a PD array has an advantage in that its reliability is enhanced as it has no movable mechanical component, and miniaturization in the construction thereof is feasible. However, it has problems that there is a limit to the extent to which cost can be reduced because of high cost of the PD array, its measurement resolving power is low because of a limited number of elements incorporated in the PD array, and so forth.
With any of the conventional wavelength measuring systems described hereinbefore, measured data need to be processed by use of a software to find a wavelength, requiring a data processor. Accordingly, in the case where these wavelength measuring systems are used for controlling a laser wavelength at a light source, the data need to be processed with a software, making them unsuitable for high-speed control of the wavelength.
Meanwhile, a wavelength measuring system called the wavelength locker, used for control of a laser wavelength at a light source, has since been developed. This wavelength measuring system has a construction wherein wavelengths are measured by a filter taking advantage of dielectric multiple layers or a spectroscopic device such as a diffraction grating.
With this type of the wavelength measuring system, however, the range of measurable wavelengths will be limited by the characteristic of a spectroscopic device (filter, and the like) in use, creating a problem that same can not be used in measuring wavelengths in a wide wavelength range. In addition, there is need for providing for spectroscopic devices dedicated for respective wavelengths, limiting the extent to which cost can be reduced.
That is, the wavelength measuring system called the wavelength locker is suitable for controlling laser wavelengths at a light source having a narrow tunable range such as the DBF--LD light source because same has high reliability without movable mechanical components, is suited for miniaturization in the construction thereof, and requires no software in processing of data. However, such a wavelength measuring system has a drawback in that it can not be used for measuring or controlling wavelengths at a light source such as the external optical cavity type tunable light source capable of varying wavelengths to 100 nm or longer.